1. Field of the Invention
The present invention relates generally to spacer grids for nuclear fuel assemblies for reducing flow-induced vibrations and, more particularly, to a spacer grid for a nuclear fuel assembly which is formed from grid strips of an improved structure, thus reducing flow-induced vibration.
2. Description of the Related Art
A nuclear reactor refers to a device that is designed to exert artificial control over the chain reaction of the nuclear fission of fissile materials and use thermal energy generated from the nuclear fission as power.
Generally, nuclear fuel that is used in a nuclear reactor is formed in such a way that enriched uranium is molded into a cylindrical pellet of a predetermined size and many pellets are inserted into fuel rods. The fuel rods constitute a nuclear fuel assembly. The nuclear fuel assembly is loaded in a core of the nuclear reactor before it is burned up in a nuclear reaction.
Referring to FIG. 1, a typical nuclear fuel assembly includes a plurality of fuel rods 10 which are located in an axial direction, a plurality of spacer grids 20 which are provided in a transverse direction of the fuel rods 10 and support the fuel rods 10, a plurality of guide thimbles 30 which are fixed to the spacer grid 20 and form a framework of the assembly, and a top nozzle 40 and a bottom nozzle 50 which respectively support upper and lower ends of the guide thimbles 30.
About 200 or more fuel rods 10 are used to form the nuclear fuel assembly. Enriched uranium is molded into a pellet of a predetermined size and installed in each fuel rod 10.
The top nozzle 40 and the bottom nozzle 50 support the upper and lower ends of the guide thimbles 30. The top nozzle 40 is provided with elastic bodies to push down an upper end of the nuclear fuel assembly, thus preventing the pressure of a coolant flowing from a lower end of the nuclear fuel assembly towards the upper end thereof from lifting up the nuclear fuel assembly. The bottom nozzle 50 supports the lower ends of the guide thimbles 30. A plurality of flow holes through which the coolant is supplied into the nuclear fuel assembly are formed in the bottom nozzle 50.
The several spacer grids 20 are arranged at predetermined intervals with respect to the axial direction of the fuel rods 10. According to the arrangement location and function, the spacer grids 20 are classified into medial spacer grids, mixing spacer grids which enhance the performance of mixing the coolant, and a protective spacer grid which filters out foreign substances.
Referring to FIG. 2, the spacer grids are commonly formed by a plurality of grid strips assembled in a lattice shape. In each spacer grid, a single fuel rod or guide thimble is disposed in each of the lattice cells.
In detail, the spacer grid 20 includes a plurality of an outer grid strip 21 which forms an outer frame of a structure, and horizontal grid strips 22 and vertical grid strips 23 which are arranged and fixed inside the outer grid strip 21 and form a lattice shape.
The fuel rods are disposed in the corresponding lattice cells 20a formed in the spacer grid 20 having the above-mentioned construction. Further, guide thimble lattice cells 20b into which the guide thimbles are inserted are formed in the spacer grid 20.
The fuel rods are assembled with the spacer grid in such a way that dimples and springs are provided on the grid strips that form the lattice cells so that the grid strips elastically support the fuel rods. Each guide thimble may be welded to the spacer grid or may be mechanically fixed thereto by a sleeve.
Meanwhile, during a process of supplying a coolant into the flow hole of the bottom nozzle 50, foreign substances, for example, pieces of metal, chips or shavings which are created when cooling equipment or piping equipment is produced, installed or repaired, may enter, along with the coolant, through the flow hole of the bottom nozzle 50.
If such foreign substances enter the assembly along with the coolant, they may damage the jacket tubes. Therefore, foreign substances along with the coolant must be prevented from entering the nuclear fuel assembly.
Among the spacer grids, the protective spacer grid that is disposed adjacent to the bottom nozzle 50 functions not only to support the fuel rods but also to filter out foreign substances which may be drawn, along with the coolant, into a nuclear reactor during the process of circulating the coolant. The protective spacer grid is also named a filtering spacer grid.
For instance, protective spacer grids were proposed in Korean Patent Registration No. 10-0898114 (date: May 11, 2009), No. 10-0918486 (date: Sep. 15, 2009), No. 10-0907634 (date: Jul. 7, 2009), No. 10-0907635 (date: Jul. 7, 2009), No. 10-0982302 (date: Sep. 8, 2010), No. 10-0927133 (date: Nov. 10, 2009) which are filed by the applicant of the present invention.
The conventional protective spacer grids are provided with filtering parts which protrude from surfaces of grid strips in bent shapes so as to filter out foreign substances from a spacer grid. In particular, the purpose of the conventional protective spacer grids is not only to improve the performance of filtering out foreign substances that pass through a flow hole of a bottom nozzle but also to minimize a pressure drop that is caused by a reduction in the cross-sectional area of the flow hole.
FIG. 3 is a perspective view illustrating a protective spacer grid according to a conventional technique. Only one of lattice cells formed from a plurality of grid strips is shown in this drawing.
Referring to FIG. 3, the conventional spacer grid 60 includes dimples 62 which are formed in the grid strips 61 to support a corresponding fuel rod, and filtering parts 63 which bend and protrude in arc-shapes so as to filter out foreign substances. A separate spring may be provided to elastically support the fuel rod, although it is not shown in the drawing.
To more reliably prevent foreign substances from entering the fuel assembly, the shape of each filtering part 63 of the protective spacer grid may be complicated, or the cross-sectional area of the filtering part 63 may be widened, thus reducing the size of the space so that foreign substances cannot pass it. However, because pressure drop of the coolant is proportional to the cross-sectional area of the spacer grid when seen in the axial direction, the shape of the filtering parts cannot be designed just to be complex, even though the complex design can enhance the performance of filtering out foreign substances.
As such, the shape and structure of the filtering parts or the dimples of the protective spacer grid are susceptible to pressure drop of a coolant. Thereby, flow-induced vibration may be caused by the coolant that is drawn into the fuel assembly at high speed.
When the natural frequency of the protective spacer grid and the frequency of flow-induced vibration attributable to turbulence formed around the edges of the grid strips of the protective spacer grid are within the same range, the protective spacer grid is subject to large vibrations because of resonance. If the protective spacer grid undergoes large vibrations for a long period of time, fatigue damage or rupture occurs, acting as a mechanism of damaging the nuclear fuel assembly.
Hence, the spacer grids, in particular, the protective spacer grid which is disposed adjacent to the bottom nozzle, must be designed to reduce flow-induced vibrations.